Species concepts

13 Pages·1997·5.5 MB·

by  Robert M Zink

R. M. Zink 97 Bull. B.O.C. 1997 117(2)
Species concepts
by Robert M. Zink
It is difficult to imagine a concept that impinges on more biological
research than that of the species. Most biological studies refer to their
subjects as members of some species (Hauser 1987). We are often
taught that species are the only units in the classificatory scheme that
exist independent of taxonomists; i.e. species are real. Given the
importance of species in theories about evolution, ecology, and
behaviour, faunal lists, and for communication ofour understanding of
biodiversity, lack of agreement about how biologists define species is
surprising.
The species debate was evident in Darwin's time (1859), and has
escaped general resolution in the last 130 years, except perhaps for the
fact that most agree that the word species derives from Latin meaning
'appearance' with a secondary meaning 'kind'. Today the literature is
replete with dijETerent definitions (Table 1). Some (e.g. Endler 1989)
suggest that diflferent species concepts are needed to study diflferent
evolutionary processes. Paleontologists must cope w4th incomplete
fossil histories and absence ofinformation on mating tendencies (Wiley
1978). Botanists must deal with reticulation, recognizing that a large
percentage ofall plant species, perhaps 50% or more, is ofhybrid origin
(Cronquist 1978). A large number of biologists claim to follow the
so-called biological species concept (BSC; Mayr 1942), in which
reproductive isolation is viewed as the crux ofspeciation. In the past 15
years, those coming from a background in phylogenetic systematics, or
'cladistics', have made substantial contributions to the debate about
species, and have penned a number of 'phylogenetic' species concepts
(Cracraft 1983, de Queiroz & Donoghue 1988, Nixon & Wheeler 1990).
Although argument over species concepts has persisted for more than
100 years, it is my thesis in this paper that the contributions from
phylogenetic systematics have substantively changed the nature of the
debate. Furthermore, it is my opinion that the long-entrenched
biological species concept is losing favour in ornithology (Zink &
McKitrick 1995, Hazevoet 1996) and elsewhere (Mallet 1995).
Comparison of the species concepts given in Table 1 is beyond the
scope of this review. Instead, I will contrast the biological species
concept and a phylogenetic alternative. The reason for doing so is that
I believe that the current debate about species concepts has come to
focus on whether to give primary emphasis to a process, such as mate
choice, or to correct representation of evolutionary patterns, such as
those recovered by direct analysis ofcharacters (Graybeal 1995). These
two conceptual positions are embodied in the BSC and a PSC,
respectively. Here I review each concept, noting some perceived
strengths and weaknesses (Tables 2, 3).
The biological species concept has a long history in ornithology
(HafTer 1992, Amadon & Short 1992, Bock 1986, Hauser 1987). A
perceived advantage of this concept is that it is 'biological'. By
R. M. Zink 98 Bull. B.O.C. 1997 117(2)
TABLE
1
Some species definitions or views on species
"No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely
what he means when he speaks ofa species." (Darwin 1859)
"Aspeciesisasetofpopulationscapableofcombiningwitheachotherbutnotwithother
similar sets of populations on the basis of aflftnity and co-direction in ecological
specialization." (Shaposhnikov 1966)
"A species is a group of organisms not itself divisible by phenetic gaps resulting from
concordantdifferencesincharacterstates (exceptformorphssuchasthoseresultingfrom
sex, caste, or age differences), but separated by such phenetic gaps from other such
groups." (Michener 1970)
"We may regard as a species (a) the smallest (most homogeneous) cluster that can be
recognized upon some given criterion as being distinct fromother such clusters, or (b) a
phenetic group ofa diversity somewhat below the subgenus category, whether or not it
contains distinct subclusters." (Sneath & Sokal 1973)
"Somit ist die Art als das Kollektiv von Lebewesen zu bestimmen, das gemeinsam eine
okologische Nische behauptet." (von Wahlert 1973)
"Species maythenbe definedas groups ofpheneticallysimilarpopulationsthathavethe
capability to interbreed, and share similar ecological characteristics." (Doyen &
Slobodchikoff1974)
"Species, then, are the most extensive units in the natural economy such that
reproductive competition occurs among their parts." (Ghiselin 1975)
"A species is a lineage (or a closely related set of lineages) which occupies an adaptive
zoneminimally differentfromanyotherlineageinits rangeandwhichevolvesseparately
from all lineages outside its range." (Van Valen 1976)
"Species are the smallest groups that are consistently and persistently distinct, and
distinguishable by ordinary means." (Cronquist 1978)
"A species is a single lineage of ancestral descendant populations of organisms which
maintains its identity from other such lineages and which has its own evolutionary
tendencies and historical fate." (Wiley 1978)
"A 'species' is merely a population or group of populations defined by one or more
apomorphous features, it is also the smallest natural aggregation of individuals with a
specifiable geographic integrity that can be defined by any current set of analytical
techniques." (Rosen 1979)
"A species is a group ofanimals or plants all ofwhich are similar enough in form to be
considered as minor variations of the same organism. Members of the group normally
interbreed and reproduce their own kind over considerable periods oftime." (Trueman
1979)
"Aspeciesisa diagnosable clusterofindividualswithinwhichthere is aparentalpattern
of ancestry and descent, beyond which there is not, and which exhibits a pattern of
phylogeneticancestryanddescentamongunits oflikekind." (Eldredge& Cracraft 1980)
"Species are simply the smallest detected samples of self-perpetuating organisms that
have unique sets ofcharacters." (Nelson & Platnick 1981)
"Each species is an internally similar part ofa phylogenetic tree." (Willis 1981)
"We can, therefore, regard as a species that most inclusive population of individual
biparental organisms which share a common fertilization system." (Paterson 1985)
"An 'evolutionary species' is a single lineage ofancestor-descendant populations which
maintains its identity from other such lineages and which has its own evolutionary
tendencies and historical fate." (Wiley 1978)
'A species is what a good taxonomist says it is." (Anon.)
R. M. Zink 99 Bull. B.O.C. 1997 117(2)
TABLE 1 continued
"At the outset I confess a disbeliefin species, as that word is commonly understood to
referto the basic taxonomic unitortothetaxonomic unitofevolution . . . Thereseemto
be no basic taxonomic units and no particular taxonomic unit of evolution . . . and as
Agassiz said in 1859 'species do not exist in nature in a different way from the higher
groups'." (Nelson 1989)
"... species as the most inclusive group of organisms having the potential for genetic
and/or demographic exchangeability." (Templeton 1989)
A species is "the smallest aggregation of populations (sexual) or lineages (asexual)
diagnosable by a unique combination of character states in comparable individuals
(semaphoronts)". (Nixon & Wheeler 1990)
Species "refer to groups ofactually or potentially interbreeding populations isolated by
intrinsic reproductive barriers from other such groups. Evidence for reproductive
barriers willinvolveconcordantgeneticdifferencesamongthepopulationsinvolved".
. . .
"Subspecies are groups ofactually orpotentially interbreeding populations phylogeneti-
cally distinguishable from, but reproductively compatible with, other such groups.
Importantly, the evidence for phylogenetic distinction must normally come from the
concordant distributions of multiple, independent, geneticallv based traits." (Avise &
Ball 1990)
"Species are groups of actually or potentially interbreeding natural populations, which
are reproductively isolated from other such groups." (Mayr 1942)
"A species is a reproductive community of populations (reproductively isolated from
others) that occupies a specific niche in nature." (Mayr 1982)
"Aspecies is the smallest diagnosable clusterofindividual organismswithinwhichthere
is a parental pattern ofancestry and descent." (Cracraft 1983)
Species are "lineages whose components (ifdistinguishable) are not incontrovertibly on
different phylogenetic trajectories (i.e. sublineages, ifdistinguishable, are reproductively
compatible), as long as these sublineages do not form a paraphyletic group in recovered
history". "The species category would representthe largestentities thathaveevolved
. . .
whoseparts, ifdistinguishable, arenotlikelyto beondifferentphylogenetictrajectories."
(Frost & Hilhs 1990)
"if a given historical group of hybridogens is persistent and is not affecting the
evolutionary trajectory ofits Mendelian ancestor (as indicated by biogeography, habitat
preferences, or genetic divergence), it should be considered a separate species." (Echelle
1990)
A species is the "smallest recognizable monophyletic or unresolved unit". (Donoghue
1985)
"Phylogenetic species canbe delimitedby aprocedure (population aggregationanalysis)
that involves a search for fixed differences among local populations, followed by
successive rounds of aggregation of populations and previously aggregated population
groups that are not distinct from each other." "descent relationships among
[phylogenetic species] must be hierarchic." (Davis & Nixon 1992)
observing birds from differentiated groups that interact in sympatry,
one can determine if they mate assortatively. Because we observe the
birds themselves choosing mates, this is deemed 'biological'. However,
there is nothing 'non-biological' about the evolution of groups of
individuals, the hallmark ofthe PSC, and the BSC cannot make a claim
to be uniquely biological. Perhaps the 'B' in 'BSC should stand for
behavioural.
R. M. Zink 100 Bull. B.O.C. 1997 117(2)
TABLE
2
Perceived strengths and weaknesses ofthe biological species concept (see Cracraft 1983,
McKitrick & Zink 1988, Zink & McKitrick 1995, Zink 1996b)
Perceivedstrengths
Reproductive isolating mechanisms objective in sympatry
Reproductive isolation=genetic closure ofa lineage
Perceivedweaknesses
Amount ofhybridization required for conspecifity is vague
Stable hybrid zones are difficult to assess
Time to fusion is potentially enormous
Allopatric populations arejudged subjectively
Evidence ofevolutionary pattern is "overruled" by actual or presumed interbreeding
Non-historical species result because hybridization is not limited to sister taxa
Reproductive isolation is an epiphenomenon, not a directly measured characteristic of
allopatric populations
TABLE
3
Perceived strengths and weaknesses ofa Phylogenetic Species Concept
Perceivedstrengths
Species limits are consistent with recovered phylogenetic patterns
Same procedures used for species limits as for higher taxonomic categories
Based directly on charactervariation, not an epiphenomenon such as mate choice
Better description ofspatial patterns ofbiodiversity
Produces units required by evolutionary theories and conservation biology
Perceivedweaknesses
Lower limits ofspecies require careful study ofvariation
Limits ofdiagnosability problematic
Too many species will result
Species might be too ephemeral
—
The crux ofthe BSC is that species are reproductively isolated they
have reached that stage in evolutionary divergence where members no
longer recognize individuals in other species as mates, or if they do,
their offspring are of reduced fitness. Conversely, taxa that are
reproductively compatible, especially if they interact in sympatry and
form a hybrid swarm, are considered to be conspecific. Thus, the
process of mate choice is accorded primary significance in determining
whether two taxa are to be considered one or two species. This view has
intuitive appeal, although I believe that the theoretical and practical
flaws with the BSC are insurmountable.
One might argue that in practice the BSC cannot be deemed of
primary significance because taxonomists working with some of the
most speciose groups actually do not appear to use it. It would be hard
to argue (see Whittemore 1993) that botanists use the BSC when
delimiting species, given the propensity of plants to hybridize.
Similarly, it is doubtful if entomologists (perhaps excluding
R. M. Zink 101 Bull. B.O.C. 1997 117(2)
lepidopterists) use the BSC, depending instead on morphological
differences to recognize most species boundaries. If botanists and
entomologists do not generally apply the BSC when describing species,
it follows that the bulk ofthe world's biodiversity is classified primarily
by a non-BSC paradigm.
Controversy has surrounded the BSC since its inception (Donoghue
1985, Mallet 1995), although many believe that it has survived the tests
of time (Coyne et al. 1988). Many problems with using the BSC are
familiar (Cracraft 1983), such as the need to speculate whether
allopatric populations are reproductively isolated (e.g. Thompson
1991). Zink & McKitrick (1995) reviewed how the significance of
hybridization has been misconstrued by proponents of the BSC. It has
been implied that hybridization might 'erase' evolved diflferences
between two taxa. However, fusion of hybridizing taxa will likely
require long time periods (Zink & McKitrick 1995). Thus, the BSC has
been criticized, rightly in my opinion, for basing species limits on what
might happen in the future rather than what has happened during the
evolutionary past (Cracraft 1983).
The primary new criticism of the BSC to emerge from the writings
of phylogenetic systematists is the recognition that reproductive
isolation often does not evolve concomitantly with characters that
delimit evolutionary taxa, especially in the early stages of divergence.
Within many biological species we can recognize separate evolutionary
groups, and analyses of their relationships can suggest a patt—ern that is
inconsistent with the pattern of reproductive compatibility i.e., two
hybridizing taxa might not be each other's nearest relatives. A 'species'
consisting of two or more groups that are not each other's nearest
relatives is unacceptable to modern systematists. In systematics terms,
reproductive compatibility is an ancestral condition, because individ-
uals in the ancestral population recognized each other as mates (Rosen
1979). To use the primitive ability to hybridize (perceived in allopatry
or documented in sympatry) as a grouping criterion for species limits,
as the BSC requires, violates—the way in which systematists unite taxa
in an evolutionary manner by possession of shared-derived (i.e.
non-ancestral) characters (Rosen 1979). Species limits based on
reproductive compatibility (BSC) or patterns of character variation
(PSC) can conflict and lead to diflferent species limits from the same
data set (Frost & Hillis 1990) (see Fox Sparrow Passerella iliaca
example below).
The above comments do not mean that advocates of a PSC consider
reproductive isolation uninteresting or unworthy of study. Indeed, one
could argue that without reproductive isolating mechanisms, the
world's species diversity would be kept low because taxa could not
become sympatric. Reproductive isolation is an inevitable but
temporally unpredictable by-product of evolutionary divergence
(McKitrick & Zink 1988). At some point in evolutionary divergence,
nearly all difiFerentiated taxa are reproductively isolated, and all
reproductively isolated taxa are likely to be 'good' phylogenetic species.
I suspect that all species concepts recognize reproductively isolated taxa
as diflferent species. However, between the time of the emergence of
R. M. Zink 102 Bull. B.O.C. 1997 117(2)
taxa on their own evolutionary trajectories (as evidenced by characters)
and their eventual reproductive isolation, the pattern of reproductive
compatibility is an unreliable predictor of historical relationships of
taxa. Although recognizing the intrinsic importance of reproductive
isolation, users of a PSC choose not to include this information in the
delimitation of species (Frost & Hillis 1990). In fact, proper study of
the evolution ofan attribute such as reproductive isolation requires first
that historical patterns among taxa are known (Brooks & McLennan
1990). The potential for non-historical groupings together with the
other often-noted problems (e.g. Donoghue 1985) leads to the in-
evitable conclusion that the BSC should be replaced with a concept that
correctly represents history.
Debate continues over how to use information on evolutionary
pattern to delimit species. For example, several phylogenetic species
concepts exist (Table 1). Davis & Nixon (1992) suggest that the phrase
*'phylogenetic species concept" is misleading because the point is to
delimit terminal taxa for phylogenetic analysis, and they describe a
process whereby phylogenetic analysis does not play a role in species
delimitation. Rather, they show how morphological or molecular
character evidence can be used alone to delimit species. Unlike the
BSC, characters are not weighted by their presumed role in a process
such as mate choice. Nonetheless, common to history-based concepts,
including the evolutionary species concept (Wiley 1978), is the (1)
rejection of reproductive compatibility as the primary criterion of
conspecific status, and (2) recognition that species can hybridize owing
to the retention ofthe ancestral ability to do so. In my opinion, the crux
of a phylogenetic species concept is to recognize groups of individuals
that have been on independent evolutionary trajectories. Evaluation of
multiple characters does not allow further subdivis—ion of such groups.
That is, a PSC attempts to recognize the status quo character analysis
reveals groups of individuals that qualify as basal evolutionary units
(Cracraft 1983, 1989). Nonetheless, ongoing debate revolves around
how^ best to recognize history at the population level (Davis & Nixon
1992, Zink & McKitrick 1995).
Several criticisms ofphylogenetic species concepts exist. Avise & Ball
(1990) suggested that with modern molecular methods, each individual
would be diagnosable, and might qualify as a separate phylogenetic
species. Indeed, if one studied individual gene genealogies (i.e. single
characters), one would likely be able to circumscribe groups of
individuals on a gene-by-gene basis that are not mutually exclusive,
historical entities (see Maddison 1995). One might then think they are
forced to recognize individual organisms as phylogenetic species to
escape the problem raised by conflicting gene genealogies. However,
the opposite trend is actually true. One uses multiple (unweighted)
characters and resolves conflicts by an explicit a priori criterion (e.g.
Davis & Nixon 1992). Species limits are set where character evidence
becomes maximally congruent. Some character conflicts might remain,
as they do in most phylogenetic studies. Thus, rather than species
being single individuals, species often will become geographically
coherent groupings of individuals. Resolution of conflicting character
R. M. Zink 103 Bull. B.O.C. 1997 117(2)
P. unalaschcensis
P. schistacea
P. megarhyncha
Figure 1. Approximate breeding distribution of four phylogenetic species of the Fox
SparrowPasserellailiaca. Phylogenetictreederivedfrompatternofrestrictionsites(Zink
1994).
distributions (e.g. gene genealogies) results in more, not less, inclusive
groupings of individuals. The mistaken belief that advocates of a
phylogenetic species concept rely on single characters to delimit species
has misled several authors (e.g. Amadon & Short 1992). Put another
way, a problem with avian subspecies is that they are often based on
single characters, and analysis of other characters can suggest different
subspecific limits (see below). No taxonomic category, species or
otherwise, will likely be based on single characters (Barrowclough
1982).
Differences between biological and phylogenetic species concepts are
illustrated by studies of the Fox Sparrow Passerella iliaca. In North
America, Fox Sparrows range over the taiga, northwest coast, and
mountainous regions of the west (Fig. 1). There is considerable
phenotypic variation, partitioned by taxonomists into 18 subspecies.
However, four basic groups exist: iliaca, unalaschcensis, megarhyncha,
and schistacea. Each of the four groups was originally recognized as a
separate species. The Fox Sparrow is currently considered a single
biological species because each group is known to hybridize with at
least one other group, although only a few hybrid specimens between
R. M. Zink 104 Bull. B.O.C. 1997 117(2)
iliaca and unalaschcensis are known (Zink & McKitrick 1995). Zink
(1994) found that each of the four groups possessed a distinct set of
DNA
mitochondrial haplotypes. Thus, both genetic and morphological
evidence—reveal four groups of individuals with separate evolutionary
histories hence, four likely phylogenetic species. (Note that molecular
analysis did not indicate a greater number of taxa than that apparent
frommorphological comparisons.) Zink (1994) suggested, however, that
furtherstudy ofmorphologywas neededto testand refine species limits.
The question of how many biological species of Fox Sparrow exist
depends on how one interprets the evidence on hybridization. An
extreme view supports one species, because at least a trickle of genes
links all parapatric forms. A moderate view might envisage three
species, iliaca, unalaschcensis, and {megarhyncha plus schistacea). The
latter grouping would obtain because of a narrow hybrid zone between
the latter two taxa (Zink 1994).
The main problem in interpreting biological species limits in the Fox
MtDNA
Sparrow concerns megarhyncha and schistacea. evidence
reveals a narrow hybrid zone between the two groups (Zink 1994). The
zone seems broader when morphometric patterns are considered. The
stability of the zone is unknown, a factor critical in interpreting
biological species limits (Mayr 1982, Zink & McKitrick 1995). If the
hybrid zone was stable, Mayr (1982) would consider schistacea and
megarhyncha to represent two species because there was an unknown
barrier to complete introgression despite random mating in the zone.
Because the zone appears to be between two non-sister taxa, it is
probably one of secondary contact (Cracraft 1989) between two
phylogenetic species that have retained the primitive ability to
hybridize. This study therefore illustrates the problem identified with
the BSC by phylogenetic systematists (e.g. Rosen 1979); other avian
examples are found in Moore et al. (1991) and Freeman & Zink (1995).
Lumping schistacea and megarhyncha into a single species would
mis-represent evolutionary history, because although they hybridize
extensively, they are not each other's nearest relatives (Fig. 1). It could
take tens of thousands of generations for significant introgression to
occur (Zink & McKitrick 1995). The PSC would recognize the status
quo, namely that these two taxa have had independent evolutionary
histories and are therefore (phylogenetic) species. Biological species
limits depend on one's interpretation of how much (or little)
hybridization is required. BSC advocates could therefore recognize 1,
2, 3, or 4 species (which should challenge the belief of those who think
that species are real entities of nature). However, a non-historical
species including only megarhyncha and schistacea, permissible under
the BSC, would be ofno intrinsic value in phylogeny studies, speciation
research, biogeography, comparative biology or conservation. I can
think of few studies that would intentionally use this classification.
Non-historical taxa, then, are the Achilles heel of the BSC.
It is worth noting that the inconsistent relationship between patterns
of evolution among populations and their reproductive isolation has
only recently been recognized, in part owing to molecular systematics.
DNA
Molecular methods, such as sequencing, can resolve evolutionary
R. M. Zink 105 Bull. B.O.C. 1997 117(2)
relationships over short evolutionary time scales, such as within
biological species. Prior to molecular analysis, relatively little attention
focused on phylogenetic relationships among, for example, subspecies
(including those in the Fox Sparrow). Molecular studies can, ofcourse,
reveal taxa 'invisible' to morphologists, but more often they elucidate
inter-relationships of closely related taxa. Thus, the BSC was invented
and used commonly before it was realized that hybridizing taxa might
not be sister taxa, and that the pattern of hybridization might
misrepresent the true pattern of evolutionary history. Now, however,
molecular phylogenetic analysis and classification (i.e. cladistic)
methods that require taxonomic boundaries to be faithful to evol-
utionary ones, mean that the BSC is not an appropriate species concept.
Given the historical usage of the BSC in ornithology, one might
wonder ifit can be salvaged. The BSC would be improved ifsubspecies
were phylogenetic species that were not reproductively isolated.
Species would be required to be consistent with recovered patterns of
history (thus, the 'Fox Sparrow' would be either one, two or four
species, but one would not accept a species consisting solely of
megarhyncha and schistacea given the pattern of history suggested in
Fig. 1). Notwithstanding potential improvements to the BSC, 'species'
would still contain variable numbers of basal evolutionary taxa, and
thus not be comparable. Also species membership of allopatric
subspecies would be judged subjectively, and still one would have to
guess whether an allopatric taxon was reproductively isolated rather
than emphasizing directly observable character variation. I think that
scientific studies require more of species concepts than can be
accomplished by re-modelling the BSC.
Implementation of a phylogenetic species concept would remove
another contentious area from avian systematics, the subspecies
category. Many practising avian taxonomists have in mind a certain
'level of distinctness' required to elevate a subspecies to species;
unfortunately, this level varies from taxonomist to taxonomist because
of subjective character weighting. Under a PSC, there is no need for
the subspecies category (McKitrick & Zink 1988). I suggest that this
would be beneficial because ofthe many avian subspecies that represent
arbitrary divisions of clines, or are based on characters not supported
by, or conflicting with, other characters. Thus, there would be a single
—
taxonomic category for basal evolutionary taxa phylogenetic species.
Mayr (1993) w^as concerned that implementation of a PSC w^ould
double the number of biological species of birds {c. 9000) recognized
worldwide. Mayr implied that this number of species would hinder
ornithological research. If this were so, botanists and entomologists
surely would be unable to conduct research. G. F. Barrowclough,
J. Cracraft & R. M. Zink (unpubl. data) estimated that c. 18,000 species
of birds exist, using the PSC. This estimate was reached by evaluating
morphological evidence that was gathered by previous avian
taxonomists for subspecies designations; in fact, many subspecies did
not qualify as phylogenetic species (see Hazevoet 1996). This number
of species can hardly be perceived as a negative aspect of a PSC
(Cracraft 1992). I suggest that it would (1) more accurately portray
R. M. Zink 106 Bull. B.O.C. 1997 117(2)
avian species diversity (Moritz 1994, Rojas 1992, Cracraft 1997), and
(2) put the assessment of avian biodiversity on a more comparable
footing with other groups of organisms.
Martin (1996) suggested that use of molecular methods would
drastically inflate the number of avian phylogenetic species, many of
which would be recognizable only with sophisticated molecular tools
(i.e. beyond field identification). Zink (1996a) suggested that the
number of new species that were diagnosable only through molecular
analyses would in fact be relatively low, and that most taxa supported
by molecular analyses also have morphological characteristics (Zink
et al. 1995). The doubling of the number of bird species mentioned
above (G. F. Barrowclough et al., unpubl. data) was based solely on
morphological criteria. At the Linnean symposium from which this
paper derives, two independent speakers estimated the number of
phylogenetic species for particular areas by examination of published
(morphological) taxonomies. Thus, I doubt that molecular analyses will
reveal many species that are 'invisible' to field workers (see the Fox
Sparrow example above), and even if they did, would we recognize as
valid components of our biodiversity only those taxa that could be
identified with binoculars and a certain level of expertise?
A phylogenetic species concept would also benefit conservation
biology (Cracraft 1997). Today, under the BSC, one needs to argue that
sometimes local populations, subspecies, or species are units that
should be targeted for conservation eflforts. In my opinion, many non-
systematists (and systematists) view this inconsistency as unscientific.
Under a phylogenetic species concept, phylogenetic species would
become the category for conservation biology. The explicit goal would
be to preserve biodiversity at its most basic spatial scale. It might be
thought that some phylogenetic species, formerly 'only' subspecies,
would be indefensible for conservation efforts. Because conservation
efforts are likely to move from the level of individual species to
communities, patterns of species diversity will be used to define
conservation entities, such as reserves that capture the bulk of areas of
endemism. Spatial patterns of species diversity are most consistently
and precisely described under a PSC.
Is the species debate important? Clearly, if different species concepts
lead to different species limits given the same data, the answer is yes.
The species debate is not a semantic battle between cladists and
evolutionary taxonomists. Many researchers are studying speciation. If
we cannot agree on a concept of species, how can speciation be
effectively studied (Zink 1996b)? If we are discussing how to preserve
biodiversity, and species are our measure, species concepts are
extremely important. If we use species in evolutionary theories, or
comparative studies (Brooks & McLennan 1990), we must be in
agreement about how they are described. We must understand
properties of species under differing concepts and determine that these
properties are what our theories and uses of species require.
Researchers should recognize the impact that different species concepts
can have on their research, and explicitly consider whether their
research programme depends on which concept is used.